1. Field of the Invention
This invention relates to color conversion coefficient preparation and color conversion image processing and in particular to color conversion image processing for converting a first n-color separation color signal containing black into a second n-color separation signal containing black, a preparation apparatus and method of color conversion coefficients used at the color conversion image processing time, a storage medium storing a program for executing such processing or color conversion coefficients, and a color conversion system for performing such processing.
2. Description of the Related Art
In print, advertising, and publishing industries, etc., often an image signal is handled as a color signal separated into four colors containing black, such as C (cyan), M (magenta), Y (yellow), and K (black). In the invention, three colors other than black are arbitrary; in the description that follows, however, CMY will be used as an example and CMYK containing black will be used.
A color signal separated into four colors is previously prepared assuming one print condition. The assumed print condition is set based on the color reproduction characteristic in a specific printer or printing machine and is a machine-dependent color signal. Thus, even if a printer accepts CMYK color signals, if it has a color reproduction characteristic different from the assumed print condition, the output result is color reproduction different from that of print under the originally assumed print condition.
In the print industry, etc., a business practice called color proofreading or color proof is conducted, namely, before a large number of sheets of printed matter ordered by a client are printed on a rotary press, etc., (real machine printing), so-called proof print is executed and client's agreement is obtained. If CMYK is digital color signal, the proof print can be executed using a marking technique other than print, for example, a thermal dye-sublimation printer, an ink jet printer, a Xerographic printer, etc. To execute proof print using a printer, it is necessary to convert CMYK four-color separation image signal into CMYK four-color separation image signal for the printer for executing the proof print so as to provide the same reproduced color as with the case where printing machine printing is executed based on CMYK four-color separation image signal separated into four colors of CMYK. Conversion from machine-dependent CMYK four-color separation image signal to machine-dependent CMYK four-color separation image signal for a printer is called CMYK-to-CMYK image conversion. The CMYK four-color separation image signal will be called electronic original. Further, for the electronic original concerning the invention, unless otherwise noted, an image (plate) for each color of CMYK is a multilevel image.
Particularly, for color proofreading, it becomes important to faithfully reproduce the state of the K plate of an electronic original, for example, black characters in a single color of K and gray in a natural image in mixed color of K and CMY or only CMY also on the reproduced mage output on a printer. This function is called K preservation. That is, faithful color reproduction and K preservation become important conditions for the color proofreading.
An electronic original is input and color proofreading can be executed on a given printer as described above. This means that not only the color proofreading, but also on-demand printing can be realized if printer output is final output. That is, if an electronic original is transmitted via various networks and is printed at the party to which the electronic original is transmitted, remote color proofreading is accomplished, and if an electronic original is transmitted via various networks and print at the party to which the electronic original is transmitted is final output, remote on-demand printing is accomplished.
To execute CMYK-to-CMYK image conversion at high speed, a color conversion mechanism is required. As the color conversion mechanism, a system using a neural network is disclosed in JP-A-2-241271 and a system using a multi-dimensional table and interpolation in combination is disclosed in JP-B-58-16180. A system using a high-order polynomial is also known. In fact, a color conversion mechanism using a system using a neural network, using a multi-dimensional table and interpolation in combination (multi-dimensional table type conversion) as described above, a mechanism (gradation conversion) for adjusting gradation separately for each color of C, M, Y, and K based on log conversion, power (γ conversion), or any other arbitrary function form including a high-order polynomial, or operations accompanying UCR (under color removal) in combination is provided. It is known that the gradation conversion uses a one-dimensional table for speeding up, and a one-dimensional table is simply called LUT (lookup table).
To use the color conversion mechanism to execute CMYK-to-CMYK image conversion, it is necessary to appropriately determine a binding coefficient if a neural network is used, a table value if the system using a multi-dimensional table and interpolation in combination, a coefficient of a polynomial if a high-order polynomial is used, a value of LUT, etc., if gradation conversion is executed, and a coefficient involved in UCR if UCR is executed. The objects to be determined will be collectively called color conversion coefficients and preparing a color conversion coefficient will be called characterization. Particularly, characterization for CMYK-to-CMYK image conversion will be called CMYK-to-CMYK color conversion and a color conversion coefficient thereof will be called a CMYK-to-CMYK color conversion coefficient.
The characterization often is accomplished in a computer program and each prepared color conversion coefficient is recorded in a file, memory, etc., together with the number of data pieces and any other information required at the read time. The record is called a profile.
An image processing apparatus receives a profile at some section, processes an electronic original in accordance with the received profile, and outputs the process result on an image output unit such as a printer for providing any desired print. Thus, generally an apparatus such as a computer for performing characterization and the image processing apparatus are separate, but the image processing apparatus itself may have the characterization function in some cases. Further, an electronic original with a profile contained is transferred, whereby the convenience of remote printing can also be improved. Thus, the remote color proof, remote printing, etc., previously described is made possible.
As previously described, it is important in CMYK-to-CMYK image conversion that reproduced color is faithful and that K preservation is accomplished. The expression “reproduced color is faithful” is used to mean that tristimulus values XYZ or color space coordinate values of a color system such as L*a*b* or L*u*v* derived from XYZ match. The values can be provided by a calorimeter. In brief, colors of print of an electronic original (preferably, color chart) as originally assumed (A output color chart) and print provided by executing CMYK-to-CMYK image conversion for the electronic original and outputting the conversion result on a different printing machine or printer (B output color chart) are measured and the color measurement values of the A and B output color charts match. This is called calorimetric match.
To provide colorimetric match, the concept of ICC Profile Format defined in International Color Consortium (ICC) is effective, namely, the concept of accomplishing colorimetric match by realizing CMYK-to-CMYK color conversion by converting from machine-dependent CMYK into machine-independent color space of a color system such as L*a*b* and converting from L*a*b* into CMYK. However, the machine-independent color space of a color system (hub space) is three dimensions and if converting from machine-dependent CMYK into hub space and converting from hub space into CMYK are simply performed, information concerning K is lost and K preservation cannot be provided because of dimension degeneration.
To provide K preservation in the CMYK-to-CMYK color conversion, for example, JP-A-10-309833 discloses a method of separately executing one-dimensional conversion from K to K and three-dimensional conversion from CMY to CMY. However, one-dimensional conversion from K to K and three-dimensional conversion from CMY to CMY are separately executed and thus the color conversion mechanism is simple, but the calorimetric match accuracy is poor, because additive property does not hold in the so-called subtractive color process like CMYK.
For example, JP-A-10-341354 discloses a method of providing n one-dimensional correspondences from K to K, then fixing ki (i=1, 2, . . . , n), providing n sets of color charts with CYM, measuring colors, and determining CYM based on the color measurement values of the color chart sets containing ki from L*a*b* found from CMYK and ki′ found from K. In this method, color chart preparation is not flexible and, for example, a disadvantage that color charts called IT8 (two types of 128 colors and 928 colors are available) widely used in the print industry, etc., cannot be used occurs. A color prediction model described later is used bit by bit and thus continuity is not guaranteed and consequently gradation level difference called pseudo contour easily occurs.
Further, JP-A-2000-78419 discloses a method of assuming four-dimensional table type conversion, finding L*a*b* from CMYK, finding K so that L* matches from K, and finding CMY from L*a*b* and K. In this method, K is found from K by a technique of L* matching described later, but K becomes excessive in a high color saturation area and consequently it may be made impossible to find matching L*a*b* found from CMYK regardless of how CMY is adjusted. This nature is also the same as in the method described in JP-A-10-341354.
To execute CMYK-to-CMYK image conversion in the four-dimensional table type color conversion, it is difficult to control the reproduction start point (where gradation starts to appear), because representative points of the four-dimensional color space of CMYK are previously stored and gap between the representative points is interpolated roughly linearly using the nearby representative point. However, if a large number of representative points are taken, the problem can be solved, but this solution is very inefficient.
Further, it is not always good to provide absolute calorimetric match and the following three cases need to be considered: The assumption is that to execute CMYK-to-CMYK image conversion, paper assumed in input and paper used in output are not necessarily the same. If input is an electronic original assuming being printed and output is an ink jet printer, a Xerographic printer, a sublimation-type heat-sensitive printer, etc., dedicated paper must be used because of restriction on the output side and paper cannot be selected as desired. If both input and output happen to use a marking technique, the same type of paper is not always available at remote location. The fact that different types of paper are used section that the state in which coloring material of ink, toner, etc., is not put, namely, L*a*b* values of white differ.
As a first example, a case where input paper has lower lightness (L* value) than output paper will be considered. To conduct calorimetric match, if input CMYK is all 0%, namely, white, some color material is put on output paper to lower the lightness. This is a first reproduction method, called complete colorimetric match. Even with complete colorimetric match, a second example is an opposite case to the above-described example, namely, if input paper has higher lightness than output paper, nothing can be performed. In this case, a predicted defect is that highlight will disappear.
Even if complete calorimetric match is provided, reproducing also white of input paper as in the first example may often be unpreferable. Likewise, as for reproduction in K single color such as black characters, if an attempt is made to conduct complete calorimetric match, the color material of input K differs from the color material of output K and thus CMY is mixed into K. JP-A-2000-78419 discloses such invention guaranteeing the single color of K when the single color K is converted into output K value. However, as for K, it is important to reproduce K single color in K single color and in addition, it is also important to represent K=100% on the input side in K=100% also on the output side, because if a printer adopts area modulation and K=100% on the input side is represented in K=80% on the output side, the image structure difference such that solid black with no structure is represented as a dot structure appears. Further, not only reproducing of K, but also reproducing of Y single color in mixed colors of other colors is unpreferable. This may also apply to M single color or C single color in some cases. Thus, often it is preferable that reproducing of a single color in the single color takes precedence over complete calorimetric match partially in all colors represented in mixed colors of CMYK. Reproducing based on complete calorimetric match and partially different from complete colorimetric match is called partial calorimetric match; this is a second reproduction method.
Even with the second reproduction method, the second example previously described in the complete calorimetric match, namely, the problem involved if input paper has higher lightness than output paper cannot be circumvented and if input paper white and output paper white largely differ, very unnatural reproducing results. In such a case, the color measurement values of input, output L*a*b*, etc., may be changed so that the color measurement values of input paper white and output paper white are made the same without unreasonably conducting complete calorimetric match or partial calorimetric match. This is a third reproduction method and is called relative calorimetric match.
In the four-dimensional table type conversion in the related art, it is difficult to reproduce color in a high color saturation area because of the K component and it is difficult to control in the vicinity of the reproducing start point as described above. Further, to aim at absolute calorimetric match, the effect of the paper white difference, the reproducibility of a single color, etc., involves a problem, as described above.